1. Field of the Invention
The present invention relates to an optical element holding structure, an exposure apparatus, and a device manufacturing method.
2. Description of the Related Art
An exposure apparatus transfers a circuit pattern formed on an original (reticle) onto a substrate (silicon wafer) by exposure. This transfer uses an optical system to image the reticle pattern on the wafer. The optical system is required to have a resolving power high enough to form a large-scale integrated circuit. This makes it necessary to minimize the optical aberration of the optical system for the exposure apparatus. In view of this, the optical system for the exposure apparatus must be excellent in the uniformity of various characteristics associated with the materials and films of optical elements such as a lens and mirror of the optical system, the processing accuracy of the optical surface shapes of the optical elements, and the assembly accuracy of the optical elements. A holding member for holding the optical element for use in the optical system is generally made of a material different from that of the optical element, such as a metal.
FIG. 6 shows a half of an optical element holding structure from the center of its optical surface, which is used for the conventional exposure apparatus.
Referring to FIG. 6, a plurality of lenses 101 and 102 are held by ring-like first holding members 103 and 104. The first holding members 103 and 104 are assembled in a cylindrical second holding member 105 and fixed to it while being pressed from above by press screw rings 106 and 107.
However, the aberration of the above-described optical element holding structure might change in response to a change in, for example, ambient temperature because the optical elements and constituent elements deform depending upon the temperature. This especially applies to an exposure apparatus using a light source with a relatively short wavelength. The exposure apparatus has optical elements made of glass materials such as quartz or fluorite, which have thermal expansion coefficients different from those of the materials of members for holding these optical elements. These optical elements and members cannot expand and contract free from any influence from each other. Consequently, the optical surfaces of these optical elements largely deform depending upon, for example, the ambient temperature. The deformation attributed to the temperature has a significant adverse influence on the aberration of the optical system.
A plurality of second holding members 105 are normally stacked on each other in the axial direction. As the second holding members 105 receive external forces upon being stacked and connected or due to other factors, the first holding members 103 and 104 which hold the optical elements receive external forces from, for example, the press screw rings 106 and 107. This deforms the optical surfaces of the optical elements, resulting in deterioration in the performance of the optical system.
To solve this problem, Japanese Patent Laid-Open No. 2001-343576 discloses an optical element holding structure which reduces deformation of the optical surface of an optical element due to, for example, an external force or a change in ambient temperature.
FIG. 7 conceptually shows an optical element holding structure to which Japanese Patent Laid-Open No. 2001-343576 is applied.
Referring to FIG. 7, a first holding member 112 holds a lens 111 as one of optical elements, and is made of a material having nearly the same thermal expansion coefficient as that of the lens 111. The lens 111 is fixed to the first holding member 112 by bonding.
A second holding member 113 coaxially holds the lens 111 and is made of a material having a thermal expansion coefficient different from that of the material of the first holding member 112. A plurality of notches are formed in circumferential portions of the first holding member 112. Elastic members 114 which form plate-like springs are inserted in these portions. The two ends of the elastic member 114 are connected to the first holding member 112, while its middle portion is connected to the second holding member 113. With this holding structure, the elastic member 114 has a low elasticity with respect to the optical element in the radial direction.
In this optical element holding structure, when the ambient temperature changes, the first holding member 112 and second holding member 113 expand or contract in different ways because they have different thermal expansion coefficients. Since the difference in thermal expansion is absorbed by bending deformation of the plate-like spring of the elastic member 114, the first holding member 112 can almost freely expand or contract.
Because the lens 111 and the first holding member 112 surrounding it have substantially the same thermal expansion coefficient, the lens 111 can deform by nearly simple expansion or simple contraction. This makes it possible to suppress any surface deformation, which may deteriorate the optical performance of the optical system.
Both the first holding member 112 and second holding member 113 are held through the elastic member 114 without being in direct contact with each other in the axial direction and radial direction. With this arrangement, deformation of the second holding member 113 due to an external force or its own weight is not directly transmitted to the first holding member 112, thereby suppressing deformation of the surface of the lens 111 upon deformation of the first holding member 112.
Unfortunately, the optical element holding structure in the above-described Japanese Patent Laid-Open No. 2001-343576 poses the following problem. That is, when the second holding member 113 deforms upon being pressed from its outer side in the radial direction, the first holding member 112 which holds the optical element 111 moves eccentrically.
FIG. 8 shows the optical element holding structure in Japanese Patent Laid-Open No. 2001-343576 when viewed from the optical axis direction of the optical element.
Reference numerals 111 to 114 as in FIG. 7 denote the same members in FIG. 8. The elastic members 114 are arranged at three points on the circumference of the first holding member 112 at an interval of 120°, and indicated by 114a, 114b, and 114c counterclockwise from the upper right member in FIG. 8. As shown in FIG. 8, assuming an arbitrary point on the optical axis of the lens 111 as the origin, an x-y-x orthogonal coordinate system in which the optical axis is the z-axis, and an r-θ-z cylindrical coordinate system in which the x-axis is θ=0 are set. Furthermore, a component of the elastic constant of the elastic member 114 on the r-θ-z coordinate system is indicated by (Kr, Kθ, Kz), and the three elastic members 114a, 114b, and 114c are assumed to have the same elastic constant.
Consider a case in which the second holding member 113 in the optical element holding structure shown in FIG. 8 has deformed into an elliptical shape upon being squeezed in the y direction as indicated by a broken line in FIG. 8 due to, for example, an external force.
Let Ky be the y component of the elastic constant of each of the elastic members 114a, 114b, and 114c. Then, the elastic member 114c satisfies Ky=Kr, and the elastic members 114a and 114b satisfy:Ky=Kr·sin 30°+Kθ·cos 30°=(½) Kr+(√ 3/2) Kθ
The sum of the y components of the elastic constants of the elastic members 114a and 114b on the plus side of the y-axis with respect to the x-axis in FIG. 8 is Kr+√3Kθ, which is larger than the y component Kr of the elastic constant of the elastic member 114c on the minus side of the y-axis. Especially this optical element holding structure has a low elasticity in the radial direction, as described above. That is, since Kr<Kθ, the difference between the y components of the elastic constants of the elastic members 114 on the plus side and minus side of the y-axis is relatively large. The elastic members 114 deform upon deformation of the second holding member 113 such that the y component of deformation of the elastic member 114c upon deformation of the second holding member 113 is larger than the overall y component of deformation of the elastic members 114a and 114b upon deformation of the second holding member 113. Then, the first holding member 112 connected to the elastic members 114 eccentrically moves in the minus direction of the y-axis relative to the second holding member 113. At the same time, the optical element 111 held by the first holding member 112 also eccentrically moves.
As described above, the optical element holding structure in Japanese Patent Laid-Open No. 2001-343576 poses the following problem. That is, when the second holding member 113 deforms upon being pressed from its outer side in the radial direction, the optical element 111 moves eccentrically. This results in deterioration in the optical performance of the optical system including the optical element holding structure.